Thank you for your good question, it also may be others question.
Actually, A sitemap tells Google which pages and files you think are important in your site, and also provides valuable information about these files.
If your site's pages are properly linked, Google can usually discover most of your site.
Proper linking means that all pages that you deem important can be reached through some form of navigation, be that your site's menu or links that you placed on pages. Even so, a sitemap can improve the crawling of larger or more complex sites, or more specialized files.
A sitemap helps search engines discover URLs on your site, but it doesn't guarantee that all the items in your sitemap will be crawled and indexed. However, in most cases, your site will benefit from having a sitemap.
About second question:
If your site offers people to publish Ads, dont "noindex" their posts, But their links need to have this Attribute: rel="nofollow", nofollow tag does not let google crawlers leaving your website.